1. Field of the Invention
The invention pertains to an novel sample configuration for performing fatigue and/or crack growth tests including complex loading with regard to the relative magnitude and waveform of the load cycles, with the ability to apply axial tension, compression, and/or torsional loading independently, potentially resulting in fully mixed-mode crack growth with non-proportional loading. While resonant (dynamic) conditions may be possible to achieve with the sample, an important object of the invention is to extend the advantages of closed-loop, non-dynamic testing to moderately high frequency ranges. Another object associated with crack growth applications is to provide a crack growth test option with a uniform constraint and/or plasticity-induced closure state across the crack front. Further objects include the ability to configure such a specimen with a stress-intensity that reduces as the crack length increases during a constant load or constant load amplitude test, and to achieve a well-defined, stable crack shape during crack growth. These and other objects, advantages and characteristic features of the present invention will become more apparent upon consideration of the following description thereof when taken in connection with the accompanying drawings depicting the same.
2. Description of the Prior Art
Fatigue and/or crack growth testing is necessary in many engineering applications where component durability and safety concerns merit the associated costs. For cyclic testing, it is often desirable to increase the frequency of such testing to more closely simulate field conditions, as is particularly true for high-cycle fatigue or crack growth threshold testing. Nevertheless, high frequency testing is also desirable if for no other reason than to reduce the duration and cost of testing. The most common fatigue test machines apply cyclic load to a sample mounted between two connection points with cyclic loading supplied by servohydraulic, or servoelectric actuation systems, and seldom exceed 100 Hz frequency capability due to inherent design limitations. However, it is not uncommon for these machines to employ closed loop load, displacement, or even crack tip stress-intensity control capable of arbitrary load waveforms and complex loading sequences, which can be very desirable in some applications. The use of these types of machines and the common samples employed for fatigue and crack growth testing are described by ASTM standards (especially ASTM E466 and E647) and is well known to those familiar with the art.
Application Ser. No. 13/031,410 pertaining to a device for cyclic loading of a test sample, is related to the present application as referenced above, and provides an extensive description of prior art test devices relevant to achieving higher test frequencies. While these devices are not specifically relevant to the present application for a test sample, it is largely true for cyclic loading devices that the higher the stiffness of the sample, the higher the possible operation frequency, particularly for non-resonant conditions involving closed-loop, arbitrary waveform operation. While treatment of the stiffness of the test machine and its elements is not uncommon in the art, the stiffness of the specimen is a less common design object. The specimen configuration associated with an ultrasonic piezoelectric dynamic system operating at 15-30 MHz as described in Gigacycle Fatigue in Mechanical Practice, by Paul C. Paris and Claude Bathias CRC Press 2004, is one exception, associated with a specific resonant testing system, and resembling a common “dogbone” style specimen.
With regard to prior art in sample geometries for fatigue and crack growth testing, the most commonly used configurations are described in the ASTM standards referenced previously, with several other potential test configurations described in stress intensity handbooks such as the Stress Analysis of Cracks Handbook, 3rd Ed (H. Tada et al, ASME press, 1997). The compact tension specimen is of particularly common usage for crack growth, but is well known to have reduced constraint in the vicinity of the intersection between the crack front and the free surface, resulting in non-uniform plasticity induced closure across the crack front for cyclic applications. Specimens with quarter circular or semicircular cracks are also popular, and have the benefit of resembling common naturally occurring crack shapes, but are also subject to free surface effects, though to a lesser degree. Free surface effects are absent in samples with a fully circular crack front, such as circular cylindrical or tubular specimens with a circumferential crack loaded in tension. However, because the stress intensity increases with crack length for these configurations (for a given load), any deviation from a truly concentric crack front creates an uneven stress intensity, with the highest stress intensity where the local crack length is longest. Thus the crack shape tends to become more irregular as the crack grows, resulting in crack front shape instability. This hampers correlation of the data with a single standard stress intensity solution, and impairs the reproducibility of results. The short rod chevron notched specimen described in U.S. Pat. No. 4,116,049, is one of the few specimens known to have a reducing stress intensity solution, which is advantageous for its use as a fracture toughness specimen, but the shape of the crack front, which is generally assumed to be straight for analysis purposes typically exhibits significant curvature. In fact, nearly all commonly used crack growth sample configurations exhibit crack shapes that typically differ from those assumed in the stress intensity solutions, introducing a degree of error in the interpretation of the results. This deficiency is not easily corrected merely by a more careful analysis of the specimen, because it is linked to free surface effects and can be material dependent.